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Crossflow microfiltration of water-in-oil emulsions using corrugated membranes
Separation and Purification Technology 22-23 (2001) 431– 441 www.elsevier.com/locate/seppur
Crossflow microfiltration of water-in-oil emulsions using corrugated membranes K. Scott *, R.J. Jachuck, D. Hall Department of Chemical and Process Engineering, Uni6ersity of Newcastle upon Tyne, Merz Court, Newcastle upon Tyne NE1 7RU, UK
Abstract The crossflow microfiltration of a 30% (w/w) water-in-oil emulsion is reported, using kerosene as the organic phase and hydrophobic PTFE membranes, with a mean pore size of 0.2 mm. The flux performance of corrugated membranes is compared with that of flat membranes and shows that a marked enhancement in flux is obtained with corrugated membranes. The influences of crossflow velocity, flow channel height and transmembrane pressure on flux rate are reported. The effect of varying the angle of corrugations on the overall flux performance is also examined. Measurement of mass transport rates at corrugated solid surfaces, using the limiting current technique, are reported and correlated in terms of channel Reynolds number. The mass transport rate and membrane flux rate show a similar dependency on crossflow velocity and Reynolds number. © 2001 Elsevier Science B.V. All rights reserved. Keywords: Crossflow membrane microfiltration; Oil–water emulsion; Corrugated; Mass transport
1. Introduction Separation of water from water-in-oil emulsions is of importance in several industries, e.g. organic solvent and vegetable oil, for the recovery of solvents and the purification of oil. The standard method for the treatment of emulsions is chemical de-emulsification followed by gravity settling. This process requires the use of a variety of chemicals and the water phase from chemical treatment needs secondary purification. This will, therefore, entail additional energy requirements and hence, higher cost. * Corresponding author. Tel.: + 44-191-2228771; fax: +44191-2225292. E-mail address: [email protected] (K. Scott).
Several effective methods have recently been developed for oil –water emulsion separation, such as coalescence of dispersion in fibrous beds, biodegradation and biotransformation of oily wastes and application of electric field to coalesce droplets. The development of membrane technologies has most recently embodied applications in the processing of emulsions. Several studies have reported that crossflow membrane microfiltration (CFMF) (and ultrafiltration) are effective processes in concentrating oil –water emulsions [1–5]. Many different approaches can be used to improve the flux in CFMF of emulsions. Turbulence promoters of various configurations have been studied [6]. However, flux enhancement is only achieved at the expense of
1383-5866/01/$ - see front matter © 2001 Elsevier Science B.V. All rights reserved. PII: S 1 3 8 3 - 5 8 6 6 ( 0 0 ) 0 0 1 8 0 - 5
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K. Scott et al. / Separation/Purification Technology 22-23 (2001) 431–441
significant increased frictional pressure drop. Most recently, attention has been drawn towards using corrugated membranes as turbulence promoters. It is proposed that corrugated membrane would promote turbulence preferentially near the wall region, causing less pressure drop in the flow channel along the membrane than other techniques, which promote turbulence in the entire feed stream. The use of corrugations of half-cylinder shape on membrane for hyperfiltration has already been reported [6,7]. This paper reports data for the crossflow microfiltration of water in oil emulsions, using kerosene as the organic phase. The influence of crossflow velocity, channel height and angle of corrugation on flux rate is reported. The effect of channel Reynolds number on mass transport rates at corrugated surfaces is also examined. The data presented here represents the first phase in designing and characterising the application of corrugated membrane modules for microfiltration, ultrafiltration and other membrane separations.
2. Experimental
2.1. Crossflow filtration The membrane used is PTFE, mean pore size of 0.2 mm (hydrophobic PTFE supplied by Gelman). The flat PTFE membranes were successfully corrugated without causing any damage or change in the membrane structure and properties. The corrugations are triangular in shape, 2 mm wide and 1 mm high. The water-in-oil emulsions were prepared by adding kerosene, containing 3% (w/w) of surfactant (sorbitane – monooleate) to a specified amount of distilled water, to form a 30% water – kerosene mixture. The emulsion was generated by mixing for 10 min at a speed of 5500 rpm. This method allowed the production of stable emulsions, with an average water droplet size of 0.3 mm and 92% of the droplet population between limits of 0.12 and 2 mm, as measured by a laser light-scattering technique. The membrane module unit, shown in Fig. 1, is composed of a two-piece machined circular plastic construction. A rectangular feed flow channel
(25× 67 mm) is drilled in the upper section. The base section of the unit includes a rectangular section (38× 79 mm) to hold the membrane. On assembly, the membrane sheet is first wetted with pure kerosene, to prevent the surfactant adsorption on the surface during filtration, and then mounted into the recess and sealed with O-ring gaskets. A permeate spacer (mesh) of the same dimensions as the membrane sheet is placed below the membrane sheet and the two sections are secured with tie bolts. The feed flows tangentially across the membrane surface through the flow channel that is, in most experiments, 2 mm deep. The permeate flows through three 1-mm holes in the base section and out through a collecting outlet. The hydraulic diameters of the feed flow channel, and the corresponding flow cross-sectional area and membrane surface area for the flat and corrugated membranes are listed in Table 1. The equipment used for carrying out the experimental work is shown schematically in Fig. 2. The feed emulsion is pumped from a 1.5 l feed vessel by means of a centrifugal re-circulation pump into a membrane module unit. Two pressure gauges are fitted at the inlet and outlet of the module unit to measure the transmembrane pressure. The transmembrane pressure (TMP) is adjusted by regulating a valve that creates a back-pressure along the membrane unit to the outlet of the pump. The filtrate (kerosene) rate is monitored by means of an electronic balance integrated with a data acquisition program running on a PC. The mass of the collected permeate is measured at 6 min intervals and graphically displayed on the screen of the PC. Constant concentration of the feed is virtually maintained by returning the permeate to the feed vessel. The retentate is re-circulated to the feed vessel, where it is continuously mixed with a variable speed stirrer. The flow rate of the feed is measured by a rotameter calibrated for a 30% water-in-kerosene emulsion.
2.2. Mass transfer measurement A cell was built to emulate the crossflow membrane unit with ‘membrane simulants’ constructed of glass plates. The corrugations of the mem-
K. Scott et al. / Separation/Purification Technology 22-23 (2001) 431–441
433
Fe(CN)6− 3 + e− Fe(CN)6− 4
(1)
branes were 1.3 mm in height, with 2 mm from peak to peak. An EG&G Princeton Applied Research VersaStat was used to conduct the electrolysis process under linear sweep voltammetry. The data was logged using EG&G Model 270 Research Electrochemistry Software. The mass transfer data was collected by utilising the limiting current behaviour of the cathodic reduction of ferricyanide:
The electrolyte consisted of the following components: 3.0 M KCO3 supporting electrolyte, 0.02 M K3Fe(CN)6 ferricyanide, to be cathodically reduced and 0.05 M K4Fe(CN)6 (ferrocyanide in excess as the reaction is reversible). A very thin nickel cathode was pressed into one of the glass plates to take the shape of the corrugations, and both it and the anode, also nickel,
Fig. 1. Schematic diagram of membrane module unit. Table 1 Membrane and flow channel dimensions Type of sheet
Flat sheet
90° corrugation to flow 45° corrugation to flow Parallel corrugation to flow
Hydraulic diameter (cm) Flow cross-section area (cm2) Membrane surface area (cm2)
0.370 0.500 16.75
0.281 0.375 20.74
0.281 0.375 21.05
0.240 0.380 20.74
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K. Scott et al. / Separation/Purification Technology 22-23 (2001) 431–441
Fig. 2. Schematic diagram of experimental test rig.
were polished with emery cloth. The back of the cathode was previously coated with insulating paint and an electrical connection established at the side edge to avoid interference to the flow pattern. Nitrogen was bubbled through the electrolyte to expel dissolved oxygen prior to the measurements. At a constant flowrate, a slow linear sweep of potentials was applied to the cell and the corresponding currents measured. As the reaction proceeded, the current increased to a maximum, the limiting current, where it remained constant as the cathodic potential increased. The limiting current plateau extended over a range of several hundred millivolts. Once the cell voltage range at which the limiting current occurred was identified, the process was repeated over a range of flow rates and in different positions over the surface of the ‘membrane’. Local mass-transfer rates were calculated directly from Faraday’s Law, stating that the current density at the electrode, j i is proportional to the reacting ion flux, Ni : si j = −nFNi
(2)
where si is the number of ions participating in the transfer of n electrons to or from the electrode. As the mass transfer coefficient k can be defined by: Ni = kC0
(3)
as the concentration of the reacting species at the cathode can be assumed to be approaching zero.
3. Results and discussion Fig. 3 show typical membrane flux rate characteristics for corrugated membranes (angle of corrugation= 45°, CFV= 1.89 m/s, TMP = 0.8 bar). Obviously, the flux rate is initially high and falls off rapidly with time of filtration. The rate of flux decline decreases with time and shows some evidence of reaching an asymptotic value. Similar flux decline behaviour is obtained for all runs. However, the transient flux decline rate varies with the operating conditions and most significantly on the applied transmembrane pressure (TMP). There are several factors that affect the flux performance of membranes during crossflow
K. Scott et al. / Separation/Purification Technology 22-23 (2001) 431–441
microfiltration of emulsions. These mainly include concentration polarisation and membrane fouling. The general effect of polarisation and fouling is to increase the resistance to flow of the continuous phase (kerosene) through the membrane.
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The membrane filtration of water-in-oil emulsions is clearly affected by fouling of membrane, as the data in Fig. 4 shows. A reduction in flux rate from : 20 to 17 l/m2 per hour, i.e. by 15% is observed in just 10 h of operation, solely due to
Fig. 3. Typical variation in flux with time. 0.2 mm PTFE membrane.
Fig. 4. The influence of fouling on membrane flux rate.
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Fig. 5. The effect of crossflow velocity on flux. TMP = 0.8 bar; temperature 25 – 28°C.
membrane fouling. This fouling is probably caused by the combination of initial pore blocking by water droplets in the emulsion and, secondly, the deposition of a cake layer on the surface. The formed cake layer is thought to change the pore size distribution, affecting both the flux rate and droplet rejection. The formation of this layer was in fact found to offer tighter rejection of water droplets. At the beginning of every run, slight permeation of dispersion was visible in the first 3 – 4 min of filtration, after which a clear permeate (almost 100% rejection) is obtained. Cleaning of membrane with solvent to remove the fouling layer, visibly apparent on the membrane, was partly successful. Typically, cleaning restored the flux to the initial value of 20 l/m2 per hour, which fell relatively quickly to : 18 l/m2 per hour in just 80 min. This suggests that solvent cleaning was effective only in removing the deposited layer but not the dispersion blocking the membrane pores.
3.1. Effects of crossflow 6elocity When the effect of crossflow velocity (CFV) was investigated, the transmembrane pressure (TMP) was maintained at 0.8 bar. In five series runs, each of 2–3 h duration, the crossflow veloc-
ity was increased step by step from 0.65 to 3.22 m/s. The tests have been performed on flat and corrugated PTFE membranes. Three different angles of corrugations with respect to flow direction were examined, 90°, 45° and parallel. The influence of crossflow velocity has been also tested for a reduced channel height, from 2 to 1.3 mm, using a PTFE membrane with parallel corrugations. Fig. 5 shows the variation in flux with crossflow velocity obtained for water-in-oil emulsions using flat and corrugated membranes. The flux rates for corrugated membranes are based on the membrane real areas, in order to isolate the effect of membrane area enhancement. It is clear in all cases that increased crossflow velocity produces a significant improvement in filtration flux. It is also clearly observed that the crossflow velocity exerts a greater effect in the presence of membrane corrugations. The bigger the angle of corrugation, the greater the flux enhancement. The influence of velocity is greatest with membrane corrugations making 90° with the emulsion flow direction. On the other hand, and as expected, parallel corrugations show no improvement in flux rates (based on real area of the membrane) in comparison with flat membranes. Increasing the crossflow velocity of feed emulsion increases the shear effect on the membrane
K. Scott et al. / Separation/Purification Technology 22-23 (2001) 431–441
surface. This helps to limit the thickness of the ‘deposit/cake’ layer formed on the membrane surface. An increase in velocity also reduces the concentration polarisation because of the increase in mass transfer of water droplets deposited from the membrane surface back into the bulk stream. Therefore, improvement in the flux is achieved by increasing the crossflow velocity of the feed. The range of crossflow velocities, 0.65 – 3.22 m/s, over which the runs were performed corresponds to Reynolds number range of :500 – 2500, i.e. the feed flow was mainly in the laminar region. The following expression [8] can be used to evaluate the mass transfer coefficient k for laminar flow in thin rectangular flow channels, k= 0.816
6uD 2 bl
0.33
(4)
where u is the average velocity of feed; D, the diffusion coefficient; b, the channel height; and L, the channel length. Under turbulent conditions (Re\2000), the Dittus –Boelter correlation [8] can be used, k = 0.023
u 0.8D 0.67 b 0.2w 0.47
(5)
where w is the kinematic viscosity of feed = v/z. It has been confirmed that the above mass transfer correlations predict the mass transfer coefficient in simple channel flow. Hence, generally flux rate, may be expected to increase by increas-
437
ing the flow velocity u. The effect is, however, much more dramatic in turbulent flow (flux varying with velocity to the 0.8 power) than in laminar flow (velocity to the power 0.33). The data in Fig. 5, for parallel channel flow along the corrugations give a reasonable correlation between flux and u 0.3 (in the approximate Re range of 500 –3000) as predicted for laminar flow. As both Eq. (4) and Eq. (5) indicate, the mass transfer coefficient (and hence flux rate) may also be increased by decreasing the channel height, b. Fig. 6 shows the effect of reducing the channel height, from 2 to 1.3 mm, on the flux rate and confirms the above argument. The effect on flux rate is, however, less dramatic when compared with the effect of crossflow velocity (variation to the 0.2 power compared with 0.8 and 0.33 for turbulent and laminar conditions, respectively). The decrease in flow channel height is thought to create higher shear rate at the membrane surface. The typical effect of reducing the channel height from 2 to 1.3 mm is an increase in flux rates by up to 40% at Re= 2200. However, this increase in flux is at the expense of a significant increase in pressure drop by :0.15 bar.
3.2. Effects of membrane corrugations Flux rates based on the projected area of the membrane in order to observe the combined effect of turbulence promotion and surface area en-
Fig. 6. The effect of channel height on flux rate. (Mahmood Figure).
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K. Scott et al. / Separation/Purification Technology 22-23 (2001) 431–441
Fig. 7. The effect of transmembrane pressure on flux for corrugated membranes. 90° angle of corrugation.
hancement, show that the influence of corrugations is markedly greater at higher feed flow velocities. Flux increases of : 30, 100 and 160% in comparison with flat membranes can be achieved for parallel, 45° and 90° angles of corrugations, respectively. This corresponds to :30% of area enhancement in comparison with flat membranes (area of : 16 cm2) and suggests that at least a 30% improvement in flux should be expected for corrugated membranes, regardless of the angle of corrugation used. However, this increase in flux rate is at the expense of higher membrane material cost. Visualisation of permeate flow suggested that rejections of water droplets close to 100% were accomplished for corrugated membranes as was the case for flat membranes. The effect of the other factor, turbulence promotion, appears to be much more predominant than enhancement of area. The mechanism of corrugations as turbulence promoters can be explained as follows: the flow of feed along the membrane corrugations result in a scouring action, which repeatedly disrupts and promotes mixing of the boundary layer formed at the membrane surface through formation of fluid ed-
dies. This leaves insufficient length of channel through which the feed can flow undisturbed. Therefore, this effect causes an increase in the permeate flux due the combined effect of thinning the boundary layer and higher mechanical shear at the wall. The mixing and turbulence promotions produced by corrugations are thought to only occur near the wall region. Thus, at the same velocity, the influence of channel height is not expected to play a major role in determining flux rate for single corrugated membranes. The effect of angle of corrugations on flux performance is clearly observed in Fig. 5. Changing the angle from 0° (parallel) up to 90° increases the flux rate. The greatest flux improvement is accomplished at an angle of 90°, simply because the highest level of turbulence promotion is reached at this angle.
3.3. Effects of transmembrane pressure Fig. 7 illustrates the typical effects of increasing the filtration transmembrane pressure (TMP), whilst keeping the crossflow velocity (CFV) constant. To some extent, flat and corrugated membranes give similar pressure effect behaviour. In
K. Scott et al. / Separation/Purification Technology 22-23 (2001) 431–441
general, the data highlight some important aspects of water-in-oil emulsion separation. An increased transmembrane pressure initially produces an improved flux. However, transmembrane pressures of higher than :0.8 bar result in slow reduction in flux. This flux decline may be connected with the increased build-up of the concentration polarisation layer. Another likely explanation for this effect is that transmembrane pressures exceeding 0.8 bar will cause the water droplets to squeeze into membrane pores resulting in pore blocking. The flux declines until the pressure is such (1.5 bar) that the droplets blocking the membrane pores break through the membrane with permeate. Permeate visualisation during experiments confirmed that permeation of the two phases (water and oil) actually occurred at a TMP just exceeding 1.5 bar. This pressure is normally called the ‘breakthrough pressure’. Further increase in TMP will obviously cause more dispersion to permeate through the membrane resulting in a marked increase in flux.
439
3.4. Mass transfer correlation Mass transfer coefficients for flow, at 90o, over the corrugations have been measured using the limiting current method. By treating the system as turbulent, the data in Fig. 8 shows a good agreement for Reynolds numbers between 200 and 2300 of: ShCOR = 0.2477 Re0.489Sc1/3
(6)
The mass transfer correlation showed that the laminar flow thought to exist at low flow rates does not fit this supposition. In fact, it is more likely that the irregular flow path caused by the corrugations gives a more turbulent flow. The theory has been explained in a publication describing the enhancement in mass transfer as being due to a self-sustaining oscillatory flow [9]. The existence of two-dimensional Tollmien – Schlichting waves causes a breakdown in the boundary layers, thereby promoting turbulence. T-S waves exist in this system due to the higher and lower velocities through the smaller and greater cross sectional areas, respectively.
Fig. 8. Correlation of mass transfer data for a corrugated membrane. Flow at 90° to corrugation.
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K. Scott et al. / Separation/Purification Technology 22-23 (2001) 431–441
Fig. 9. Correlation of membrane flux with Reynolds number for water in oil emulsion.
The membrane flux data of Fig. 5 is plotted as a log –log plot against Reynolds number in Fig. 9. On the figure is a line (solid) correlating the data with Re0.49 in accordance with the mass transfer correlation above for the region of Re\ 1000. There is a reasonable correlation of the data which indicates that measurement of mass transport behaviour at corrugated surfaces can give a reasonable indication of the likely flux rates expected in crossflow filtration. However, the flux data also gives a moderate correlation with Re0.8
in accord with the effect of Re on mass transport coefficient predicted by Eq. (5) above for turbulent flow. Thus, with the limited amount of data available from this study, no concrete conclusions can be made with regard to the relationship between flux and mass transport coefficient and the effect of Reynolds number. In addition, the mass transfer correlation has been obtained for single phase flow of fluids of relatively low viscosity, which are quite different to that used in the crossflow filtration.
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4. Conclusions
Acknowledgements
Experiments on flat and corrugated membranes have shown that increases in crossflow velocity and decrease in flow channel height result in an improved permeate flux caused by increased shear effects on the membrane surface. The use of corrugated membranes, however, proved to enhance the flux in a more efficient way by preferentially promoting turbulence near the membrane wall region, repeatedly mixing the boundary layer and hence, reducing the concentration polarisation. The angle of corrugation was also found to have a marked effect on the flux. Flux increases of : 30, 100 and 160% in comparison with flat membranes was achieved for parallel, 45° and 90° of corrugation, respectively. It was illustrated that corrugations with angles of 45° and 90°, can lead to a reduction in energy consumption of up to 80 and 88%, respectively. The shape of the corrugations and distance between corrugations have a marked effect on the level of turbulence in the boundary layer.
Gelman Sciences for the supply of PTFE membrane. B. Hu and A.J. Mahmood for assistance with experimental details of the programme.
.
References [1] R. Villarroel Lopez, S. Elmaleh, N. Ghaffor, J. Membr. Sci. 102 (1995) 55 – 64. [2] A.B. Koltuniewicz, R.W. Field, T.C. Arnot, J. Membr. Sci. 102 (1995) 193– 207. [3] A. Chinen, H. Ohya, Macromol. Symp. 118 (1997) 413– 418. [4] K. Scott, A. Adhamy, W. Atteck, C. Davidson, Water Res. 28 (2) (1994) 137– 145. [5] A.B. Koltuniewicz, R.W. Field, Desalination 105 (1996) 79 – 89. [6] M.J. van der Waal, I.G. Racz, J. Membr. Sci. 40 (1989) 243– 260. [7] I.G. Racz, J.G. Wassink, R. Klaassen, Desalination 60 (1986) 213– 222. [8] M.S. Le, K.L. Gollan, J. Membr. Sci. 40 (1989) 231– 242. [9] T. Nishmura, K. Kunitsugu, H. Nakagiri, Heat Trans. Jpn. Res. 27 (7) (1998) 522.
Crossflow microfiltration of water-in-oil emulsions using corrugated membranes K. Scott *, R.J. Jachuck, D. Hall Department of Chemical and Process Engineering, Uni6ersity of Newcastle upon Tyne, Merz Court, Newcastle upon Tyne NE1 7RU, UK
Abstract The crossflow microfiltration of a 30% (w/w) water-in-oil emulsion is reported, using kerosene as the organic phase and hydrophobic PTFE membranes, with a mean pore size of 0.2 mm. The flux performance of corrugated membranes is compared with that of flat membranes and shows that a marked enhancement in flux is obtained with corrugated membranes. The influences of crossflow velocity, flow channel height and transmembrane pressure on flux rate are reported. The effect of varying the angle of corrugations on the overall flux performance is also examined. Measurement of mass transport rates at corrugated solid surfaces, using the limiting current technique, are reported and correlated in terms of channel Reynolds number. The mass transport rate and membrane flux rate show a similar dependency on crossflow velocity and Reynolds number. © 2001 Elsevier Science B.V. All rights reserved. Keywords: Crossflow membrane microfiltration; Oil–water emulsion; Corrugated; Mass transport
1. Introduction Separation of water from water-in-oil emulsions is of importance in several industries, e.g. organic solvent and vegetable oil, for the recovery of solvents and the purification of oil. The standard method for the treatment of emulsions is chemical de-emulsification followed by gravity settling. This process requires the use of a variety of chemicals and the water phase from chemical treatment needs secondary purification. This will, therefore, entail additional energy requirements and hence, higher cost. * Corresponding author. Tel.: + 44-191-2228771; fax: +44191-2225292. E-mail address: [email protected] (K. Scott).
Several effective methods have recently been developed for oil –water emulsion separation, such as coalescence of dispersion in fibrous beds, biodegradation and biotransformation of oily wastes and application of electric field to coalesce droplets. The development of membrane technologies has most recently embodied applications in the processing of emulsions. Several studies have reported that crossflow membrane microfiltration (CFMF) (and ultrafiltration) are effective processes in concentrating oil –water emulsions [1–5]. Many different approaches can be used to improve the flux in CFMF of emulsions. Turbulence promoters of various configurations have been studied [6]. However, flux enhancement is only achieved at the expense of
1383-5866/01/$ - see front matter © 2001 Elsevier Science B.V. All rights reserved. PII: S 1 3 8 3 - 5 8 6 6 ( 0 0 ) 0 0 1 8 0 - 5
432
K. Scott et al. / Separation/Purification Technology 22-23 (2001) 431–441
significant increased frictional pressure drop. Most recently, attention has been drawn towards using corrugated membranes as turbulence promoters. It is proposed that corrugated membrane would promote turbulence preferentially near the wall region, causing less pressure drop in the flow channel along the membrane than other techniques, which promote turbulence in the entire feed stream. The use of corrugations of half-cylinder shape on membrane for hyperfiltration has already been reported [6,7]. This paper reports data for the crossflow microfiltration of water in oil emulsions, using kerosene as the organic phase. The influence of crossflow velocity, channel height and angle of corrugation on flux rate is reported. The effect of channel Reynolds number on mass transport rates at corrugated surfaces is also examined. The data presented here represents the first phase in designing and characterising the application of corrugated membrane modules for microfiltration, ultrafiltration and other membrane separations.
2. Experimental
2.1. Crossflow filtration The membrane used is PTFE, mean pore size of 0.2 mm (hydrophobic PTFE supplied by Gelman). The flat PTFE membranes were successfully corrugated without causing any damage or change in the membrane structure and properties. The corrugations are triangular in shape, 2 mm wide and 1 mm high. The water-in-oil emulsions were prepared by adding kerosene, containing 3% (w/w) of surfactant (sorbitane – monooleate) to a specified amount of distilled water, to form a 30% water – kerosene mixture. The emulsion was generated by mixing for 10 min at a speed of 5500 rpm. This method allowed the production of stable emulsions, with an average water droplet size of 0.3 mm and 92% of the droplet population between limits of 0.12 and 2 mm, as measured by a laser light-scattering technique. The membrane module unit, shown in Fig. 1, is composed of a two-piece machined circular plastic construction. A rectangular feed flow channel
(25× 67 mm) is drilled in the upper section. The base section of the unit includes a rectangular section (38× 79 mm) to hold the membrane. On assembly, the membrane sheet is first wetted with pure kerosene, to prevent the surfactant adsorption on the surface during filtration, and then mounted into the recess and sealed with O-ring gaskets. A permeate spacer (mesh) of the same dimensions as the membrane sheet is placed below the membrane sheet and the two sections are secured with tie bolts. The feed flows tangentially across the membrane surface through the flow channel that is, in most experiments, 2 mm deep. The permeate flows through three 1-mm holes in the base section and out through a collecting outlet. The hydraulic diameters of the feed flow channel, and the corresponding flow cross-sectional area and membrane surface area for the flat and corrugated membranes are listed in Table 1. The equipment used for carrying out the experimental work is shown schematically in Fig. 2. The feed emulsion is pumped from a 1.5 l feed vessel by means of a centrifugal re-circulation pump into a membrane module unit. Two pressure gauges are fitted at the inlet and outlet of the module unit to measure the transmembrane pressure. The transmembrane pressure (TMP) is adjusted by regulating a valve that creates a back-pressure along the membrane unit to the outlet of the pump. The filtrate (kerosene) rate is monitored by means of an electronic balance integrated with a data acquisition program running on a PC. The mass of the collected permeate is measured at 6 min intervals and graphically displayed on the screen of the PC. Constant concentration of the feed is virtually maintained by returning the permeate to the feed vessel. The retentate is re-circulated to the feed vessel, where it is continuously mixed with a variable speed stirrer. The flow rate of the feed is measured by a rotameter calibrated for a 30% water-in-kerosene emulsion.
2.2. Mass transfer measurement A cell was built to emulate the crossflow membrane unit with ‘membrane simulants’ constructed of glass plates. The corrugations of the mem-
K. Scott et al. / Separation/Purification Technology 22-23 (2001) 431–441
433
Fe(CN)6− 3 + e− Fe(CN)6− 4
(1)
branes were 1.3 mm in height, with 2 mm from peak to peak. An EG&G Princeton Applied Research VersaStat was used to conduct the electrolysis process under linear sweep voltammetry. The data was logged using EG&G Model 270 Research Electrochemistry Software. The mass transfer data was collected by utilising the limiting current behaviour of the cathodic reduction of ferricyanide:
The electrolyte consisted of the following components: 3.0 M KCO3 supporting electrolyte, 0.02 M K3Fe(CN)6 ferricyanide, to be cathodically reduced and 0.05 M K4Fe(CN)6 (ferrocyanide in excess as the reaction is reversible). A very thin nickel cathode was pressed into one of the glass plates to take the shape of the corrugations, and both it and the anode, also nickel,
Fig. 1. Schematic diagram of membrane module unit. Table 1 Membrane and flow channel dimensions Type of sheet
Flat sheet
90° corrugation to flow 45° corrugation to flow Parallel corrugation to flow
Hydraulic diameter (cm) Flow cross-section area (cm2) Membrane surface area (cm2)
0.370 0.500 16.75
0.281 0.375 20.74
0.281 0.375 21.05
0.240 0.380 20.74
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K. Scott et al. / Separation/Purification Technology 22-23 (2001) 431–441
Fig. 2. Schematic diagram of experimental test rig.
were polished with emery cloth. The back of the cathode was previously coated with insulating paint and an electrical connection established at the side edge to avoid interference to the flow pattern. Nitrogen was bubbled through the electrolyte to expel dissolved oxygen prior to the measurements. At a constant flowrate, a slow linear sweep of potentials was applied to the cell and the corresponding currents measured. As the reaction proceeded, the current increased to a maximum, the limiting current, where it remained constant as the cathodic potential increased. The limiting current plateau extended over a range of several hundred millivolts. Once the cell voltage range at which the limiting current occurred was identified, the process was repeated over a range of flow rates and in different positions over the surface of the ‘membrane’. Local mass-transfer rates were calculated directly from Faraday’s Law, stating that the current density at the electrode, j i is proportional to the reacting ion flux, Ni : si j = −nFNi
(2)
where si is the number of ions participating in the transfer of n electrons to or from the electrode. As the mass transfer coefficient k can be defined by: Ni = kC0
(3)
as the concentration of the reacting species at the cathode can be assumed to be approaching zero.
3. Results and discussion Fig. 3 show typical membrane flux rate characteristics for corrugated membranes (angle of corrugation= 45°, CFV= 1.89 m/s, TMP = 0.8 bar). Obviously, the flux rate is initially high and falls off rapidly with time of filtration. The rate of flux decline decreases with time and shows some evidence of reaching an asymptotic value. Similar flux decline behaviour is obtained for all runs. However, the transient flux decline rate varies with the operating conditions and most significantly on the applied transmembrane pressure (TMP). There are several factors that affect the flux performance of membranes during crossflow
K. Scott et al. / Separation/Purification Technology 22-23 (2001) 431–441
microfiltration of emulsions. These mainly include concentration polarisation and membrane fouling. The general effect of polarisation and fouling is to increase the resistance to flow of the continuous phase (kerosene) through the membrane.
435
The membrane filtration of water-in-oil emulsions is clearly affected by fouling of membrane, as the data in Fig. 4 shows. A reduction in flux rate from : 20 to 17 l/m2 per hour, i.e. by 15% is observed in just 10 h of operation, solely due to
Fig. 3. Typical variation in flux with time. 0.2 mm PTFE membrane.
Fig. 4. The influence of fouling on membrane flux rate.
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Fig. 5. The effect of crossflow velocity on flux. TMP = 0.8 bar; temperature 25 – 28°C.
membrane fouling. This fouling is probably caused by the combination of initial pore blocking by water droplets in the emulsion and, secondly, the deposition of a cake layer on the surface. The formed cake layer is thought to change the pore size distribution, affecting both the flux rate and droplet rejection. The formation of this layer was in fact found to offer tighter rejection of water droplets. At the beginning of every run, slight permeation of dispersion was visible in the first 3 – 4 min of filtration, after which a clear permeate (almost 100% rejection) is obtained. Cleaning of membrane with solvent to remove the fouling layer, visibly apparent on the membrane, was partly successful. Typically, cleaning restored the flux to the initial value of 20 l/m2 per hour, which fell relatively quickly to : 18 l/m2 per hour in just 80 min. This suggests that solvent cleaning was effective only in removing the deposited layer but not the dispersion blocking the membrane pores.
3.1. Effects of crossflow 6elocity When the effect of crossflow velocity (CFV) was investigated, the transmembrane pressure (TMP) was maintained at 0.8 bar. In five series runs, each of 2–3 h duration, the crossflow veloc-
ity was increased step by step from 0.65 to 3.22 m/s. The tests have been performed on flat and corrugated PTFE membranes. Three different angles of corrugations with respect to flow direction were examined, 90°, 45° and parallel. The influence of crossflow velocity has been also tested for a reduced channel height, from 2 to 1.3 mm, using a PTFE membrane with parallel corrugations. Fig. 5 shows the variation in flux with crossflow velocity obtained for water-in-oil emulsions using flat and corrugated membranes. The flux rates for corrugated membranes are based on the membrane real areas, in order to isolate the effect of membrane area enhancement. It is clear in all cases that increased crossflow velocity produces a significant improvement in filtration flux. It is also clearly observed that the crossflow velocity exerts a greater effect in the presence of membrane corrugations. The bigger the angle of corrugation, the greater the flux enhancement. The influence of velocity is greatest with membrane corrugations making 90° with the emulsion flow direction. On the other hand, and as expected, parallel corrugations show no improvement in flux rates (based on real area of the membrane) in comparison with flat membranes. Increasing the crossflow velocity of feed emulsion increases the shear effect on the membrane
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surface. This helps to limit the thickness of the ‘deposit/cake’ layer formed on the membrane surface. An increase in velocity also reduces the concentration polarisation because of the increase in mass transfer of water droplets deposited from the membrane surface back into the bulk stream. Therefore, improvement in the flux is achieved by increasing the crossflow velocity of the feed. The range of crossflow velocities, 0.65 – 3.22 m/s, over which the runs were performed corresponds to Reynolds number range of :500 – 2500, i.e. the feed flow was mainly in the laminar region. The following expression [8] can be used to evaluate the mass transfer coefficient k for laminar flow in thin rectangular flow channels, k= 0.816
6uD 2 bl
0.33
(4)
where u is the average velocity of feed; D, the diffusion coefficient; b, the channel height; and L, the channel length. Under turbulent conditions (Re\2000), the Dittus –Boelter correlation [8] can be used, k = 0.023
u 0.8D 0.67 b 0.2w 0.47
(5)
where w is the kinematic viscosity of feed = v/z. It has been confirmed that the above mass transfer correlations predict the mass transfer coefficient in simple channel flow. Hence, generally flux rate, may be expected to increase by increas-
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ing the flow velocity u. The effect is, however, much more dramatic in turbulent flow (flux varying with velocity to the 0.8 power) than in laminar flow (velocity to the power 0.33). The data in Fig. 5, for parallel channel flow along the corrugations give a reasonable correlation between flux and u 0.3 (in the approximate Re range of 500 –3000) as predicted for laminar flow. As both Eq. (4) and Eq. (5) indicate, the mass transfer coefficient (and hence flux rate) may also be increased by decreasing the channel height, b. Fig. 6 shows the effect of reducing the channel height, from 2 to 1.3 mm, on the flux rate and confirms the above argument. The effect on flux rate is, however, less dramatic when compared with the effect of crossflow velocity (variation to the 0.2 power compared with 0.8 and 0.33 for turbulent and laminar conditions, respectively). The decrease in flow channel height is thought to create higher shear rate at the membrane surface. The typical effect of reducing the channel height from 2 to 1.3 mm is an increase in flux rates by up to 40% at Re= 2200. However, this increase in flux is at the expense of a significant increase in pressure drop by :0.15 bar.
3.2. Effects of membrane corrugations Flux rates based on the projected area of the membrane in order to observe the combined effect of turbulence promotion and surface area en-
Fig. 6. The effect of channel height on flux rate. (Mahmood Figure).
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Fig. 7. The effect of transmembrane pressure on flux for corrugated membranes. 90° angle of corrugation.
hancement, show that the influence of corrugations is markedly greater at higher feed flow velocities. Flux increases of : 30, 100 and 160% in comparison with flat membranes can be achieved for parallel, 45° and 90° angles of corrugations, respectively. This corresponds to :30% of area enhancement in comparison with flat membranes (area of : 16 cm2) and suggests that at least a 30% improvement in flux should be expected for corrugated membranes, regardless of the angle of corrugation used. However, this increase in flux rate is at the expense of higher membrane material cost. Visualisation of permeate flow suggested that rejections of water droplets close to 100% were accomplished for corrugated membranes as was the case for flat membranes. The effect of the other factor, turbulence promotion, appears to be much more predominant than enhancement of area. The mechanism of corrugations as turbulence promoters can be explained as follows: the flow of feed along the membrane corrugations result in a scouring action, which repeatedly disrupts and promotes mixing of the boundary layer formed at the membrane surface through formation of fluid ed-
dies. This leaves insufficient length of channel through which the feed can flow undisturbed. Therefore, this effect causes an increase in the permeate flux due the combined effect of thinning the boundary layer and higher mechanical shear at the wall. The mixing and turbulence promotions produced by corrugations are thought to only occur near the wall region. Thus, at the same velocity, the influence of channel height is not expected to play a major role in determining flux rate for single corrugated membranes. The effect of angle of corrugations on flux performance is clearly observed in Fig. 5. Changing the angle from 0° (parallel) up to 90° increases the flux rate. The greatest flux improvement is accomplished at an angle of 90°, simply because the highest level of turbulence promotion is reached at this angle.
3.3. Effects of transmembrane pressure Fig. 7 illustrates the typical effects of increasing the filtration transmembrane pressure (TMP), whilst keeping the crossflow velocity (CFV) constant. To some extent, flat and corrugated membranes give similar pressure effect behaviour. In
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general, the data highlight some important aspects of water-in-oil emulsion separation. An increased transmembrane pressure initially produces an improved flux. However, transmembrane pressures of higher than :0.8 bar result in slow reduction in flux. This flux decline may be connected with the increased build-up of the concentration polarisation layer. Another likely explanation for this effect is that transmembrane pressures exceeding 0.8 bar will cause the water droplets to squeeze into membrane pores resulting in pore blocking. The flux declines until the pressure is such (1.5 bar) that the droplets blocking the membrane pores break through the membrane with permeate. Permeate visualisation during experiments confirmed that permeation of the two phases (water and oil) actually occurred at a TMP just exceeding 1.5 bar. This pressure is normally called the ‘breakthrough pressure’. Further increase in TMP will obviously cause more dispersion to permeate through the membrane resulting in a marked increase in flux.
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3.4. Mass transfer correlation Mass transfer coefficients for flow, at 90o, over the corrugations have been measured using the limiting current method. By treating the system as turbulent, the data in Fig. 8 shows a good agreement for Reynolds numbers between 200 and 2300 of: ShCOR = 0.2477 Re0.489Sc1/3
(6)
The mass transfer correlation showed that the laminar flow thought to exist at low flow rates does not fit this supposition. In fact, it is more likely that the irregular flow path caused by the corrugations gives a more turbulent flow. The theory has been explained in a publication describing the enhancement in mass transfer as being due to a self-sustaining oscillatory flow [9]. The existence of two-dimensional Tollmien – Schlichting waves causes a breakdown in the boundary layers, thereby promoting turbulence. T-S waves exist in this system due to the higher and lower velocities through the smaller and greater cross sectional areas, respectively.
Fig. 8. Correlation of mass transfer data for a corrugated membrane. Flow at 90° to corrugation.
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Fig. 9. Correlation of membrane flux with Reynolds number for water in oil emulsion.
The membrane flux data of Fig. 5 is plotted as a log –log plot against Reynolds number in Fig. 9. On the figure is a line (solid) correlating the data with Re0.49 in accordance with the mass transfer correlation above for the region of Re\ 1000. There is a reasonable correlation of the data which indicates that measurement of mass transport behaviour at corrugated surfaces can give a reasonable indication of the likely flux rates expected in crossflow filtration. However, the flux data also gives a moderate correlation with Re0.8
in accord with the effect of Re on mass transport coefficient predicted by Eq. (5) above for turbulent flow. Thus, with the limited amount of data available from this study, no concrete conclusions can be made with regard to the relationship between flux and mass transport coefficient and the effect of Reynolds number. In addition, the mass transfer correlation has been obtained for single phase flow of fluids of relatively low viscosity, which are quite different to that used in the crossflow filtration.
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4. Conclusions
Acknowledgements
Experiments on flat and corrugated membranes have shown that increases in crossflow velocity and decrease in flow channel height result in an improved permeate flux caused by increased shear effects on the membrane surface. The use of corrugated membranes, however, proved to enhance the flux in a more efficient way by preferentially promoting turbulence near the membrane wall region, repeatedly mixing the boundary layer and hence, reducing the concentration polarisation. The angle of corrugation was also found to have a marked effect on the flux. Flux increases of : 30, 100 and 160% in comparison with flat membranes was achieved for parallel, 45° and 90° of corrugation, respectively. It was illustrated that corrugations with angles of 45° and 90°, can lead to a reduction in energy consumption of up to 80 and 88%, respectively. The shape of the corrugations and distance between corrugations have a marked effect on the level of turbulence in the boundary layer.
Gelman Sciences for the supply of PTFE membrane. B. Hu and A.J. Mahmood for assistance with experimental details of the programme.
.
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